Optical device for the contactless measurement of distance of a light source

ABSTRACT

The invention concerns an optical device for the contactless measurement of the distance of a light source ( 4 ) comprising optical detecting means ( 7 ) consisting of elementary sensors and further comprising: a set of N imaging means (SP 1 , SP 2 , SP 3 ), N≧3, that enable image acquisition of the light source on a plane in the proximity of the detecting means thereby forming with said means a set of hot spots, each spot spreading over at least two elementary sensors; a circuit for computing, on the basis of the relative positions of at least three hot spots, at least one characteristic parameter of the distance from the light source whereof the accuracy depends on N number of hot spots. The measuring device is characterized in that it is designed for industrial measurements (of dimensions, texture, position, vibrations, displacement).

The present invention relates to an optical device for contactlessmeasurement of distance from a light source. For example, it applies inthe industrial field to non-destructive dimensional checking(measurement of levels or of thickness, checking the profile ofcomponents, cartography, robot position-fixing).

Among optical devices for contactless distance measurement, thetriangulation systems, which are very widespread in the industrialfield, form low-cost and easy-to-use devices. The principle of themeasurement of the distance from an object to the device is thecalculation of the angle at which this object is seen. A point or lineof light projected off-axis (that is to say along an axis differing fromthe optical axis of the system) onto the object are re-imaged onto aplane in the vicinity of that of a detector forming a light spot; inthis type of device, the object is regarded as being a point; theposition of the spot on the detector, defined, for example, by thecentre of gravity of the distribution of the brightness, is proportionalto the tangent of the angle at which the projected point or the line isseen. This device carries out no image recognition and does not need tohave good resolution. In contrast, it works with optics of low numericalaperture and exhibits a good depth of measurement field. One of the maindrawbacks of this method is the off-axis projection by an illuminationsystem of the light source, which has the effect of creating shadowregions which cannot be measured, in particular when the measured objectcontains high spatial frequencies (that is to say steep slopes).Moreover, the dimensions of such a system increase markedly with theworking distance, because of the angle necessary between thetransmission channel and the reception channel.

The devices based on the principle of stereoscopy, developed, forexample, for motor vehicle applications, are superior. They employ twooptical systems associated with two optical detection means separated bya distance L, each optical system forming one image of an object, thedistance from the object then being determined by the measurement of theseparation λ between the two images given by the two detection means. Inthis type of device, the accuracy of measurement of the distance isdirectly proportional to the distance L, which dictates having twoseparate detection means sufficiently far apart from one another. Thatbeing so, this type of device is bulky and constricting in use, since itrequires very high stability as to the relative position of thedetectors. In order to gain in accuracy, some of these devices possessimproved image-processing and recognition algorithms, allowing thembetter to assess the separation λ (see, for example, the patent EP 0 558026). These devices require optics with high numerical aperture (lowfocal-length/diameter ratio) in order to have very good opticalresolution, and therefore exhibit a shallow depth of measurement fieldwhich dictates that they work with field-adapting objective lenses.

Other distance-measuring devices are based on the investigation of thedefocusing of the image point of an object point through a main lenswhen the object point is shifted on either side of a nominal position(see for example the IEEE document “Transactions on pattern analysis andmachine intelligence”, vol. 14, No 2, Febuary 1992, pages 99-106,Adelson et al.).

The invention proposes a compact distance-measuring optical device, withexcellent accuracy, exhibiting a good depth of measurement field andcapable of working on the axis of the device. It includes a set ofimaging means forming, from one light source, a set of light spots on adetection means, the distance from the source being determined on thebasis of the relative position of the spots. These imaging means arepositioned in a plane close to the pupillary plane, each meansconstituting a sub-pupil. In the device according to the invention, theaccuracy on the distance depends on the number of light spots. Thehigher this number, the more accurate the measurement; hence, withsufficiently small imaging means, for example micro-lenses, the devicerequires only a single detection means, for example a linear array or amatrix of detectors.

More precisely, the invention relates to a device for contactlessmeasurement of distance from a light source including an opticaldetection means formed by elementary detectors and a set of N imagingmeans (6), N≧3, characterised in that:

the imaging means make it possible to image the light source on a planein the vicinity of that of the detection means, thus forming, on thesaid means, a set of at least three light spots, each spot being spreadover at least two elementary detectors;

the device further includes a calculating circuit making it possible, onthe basis of the relative positions of at least three light spots, tocalculate at least one parameter characteristic of the distance from thelight source to the device with an accuracy depending on the number N oflight spots.

Advantageously, the calculating circuit may include a calibration tablecontaining at least one calibration parameter the values of which aredetermined for certain distances from the light source and compares thecharacteristic parameter with the values of the said calibrationparameter. According to one operating mode, the device may furtherincludes a circuit for spatial position-fixing of the light spots on thedetection means, determining, for each light spot, its position withrespect to a reference origin. The device according to the inventionexhibits the advantage of being compact, simple to use, and fast sinceit employs simple calculating algorithms; it works with imaging meanswith low numerical aperture since it does not require optical resolutionand thus exhibits a great depth of field without it being necessary touse a field-adapting objective lens, even if that were possible.Moreover, such a device can operate on the axis, its large number ofimaging means limiting the shadow regions which cannot be measured.

Further advantages and characteristics of the invention will emerge onreading the description illustrated by the following figures:

FIG. 1 represents a first example of a device according to theinvention.

FIG. 2 represents a variant of the device employing an illuminationsystem placed off-axis and a shaping objective lens.

FIG. 3 shows the profile of the light spots in an example deviceaccording to the invention.

FIGS. 4A, 4B, 4C illustrate principles for calculating the distanceaccording to one operating mode of the invention.

FIG. 5 represents, in diagrammatic form, a possible calculating mode forthis operating mode.

FIGS. 6A and 6B represent a variant of the device in a particularconfiguration in which the secondary light source is a line.

FIGS. 1 and 2 illustrate examples of devices according to the inventionmaking it possible to measure the distance from a light source 4. In thefirst place, the source is regarded as being point-like or nearlypoint-like. In general, this source may be natural (a fluorescentmolecule for example), or may be a light source consisting of anelementary surface of an object illuminated with the aid of anillumination system, the source then being called secondary lightsource. Hence, the devices illustrated in FIGS. 1 and 2 include anillumination device (1, 2) making it possible, for example, to project apoint of light onto the object for which it is desired to measure thedistance to the system. This point of light serves as a secondary lightsource 4. The light emitter 1 used for the illumination may be a laserdiode coupled with an objective lens 2 providing focusing at the nominalworking distance. This focusing may also be done along a line as isillustrated by FIGS. 6A and 6B and will be explained later. Theillumination device may be aligned (FIG. 1) with the optical axis of thereception channel of the system 9; in this case, collinearity makes itpossible to overcome the risks of non-measurable shadow areas. Thealigning of the illumination system on the axis may be carried out usinga separating plate 3. The illumination device may be off-axis (FIG. 2),which simplifies the system; this configuration may be used particularlyfor “long range” applications for which the shadow regions arenegligible. There is no angular constraint on positioning of theillumination system in a non-collinear configuration, in contrast to thetriangulation systems of the prior art.

If the object is diffusing, the light source sends back the light in alldirections, some of which comes back to the reception system. If theobject is reflecting, the light source sends back the light in a conethe aperture of which is defined by the illumination device, and thedirection of the axis of which depends on the orientation of the object.It is consequently necessary to provide for suitable matching: either aphysical orientation of the object, or a high optical aperture of theillumination device as is encountered in microscopy.

The device according to the invention includes a detection means 7 alsocalled detector, formed by elementary detectors. It further includes aset of N imaging means 6A, N≧3, such that they make it possible to imagethe light source 4 on a plane in the vicinity of that of the detectionmeans 7, thus forming, on the said means, a set of at least three lightspots 8, each spot being spread over at least two elementary detectors.Placed in a pupillary plane, the imaging means make it possible to breakdown the beam originating from the light source into the same number ofsub-pupils.

In the example illustrated in FIGS. 1 and 2, the detection means is alinear array of elementary detectors (detectors arranged into a line)and the imaging means consist of a linear array of substantiallyidentical micro-lenses arranged side-by-side with a substantiallyconstant pitch. We will see in what follows that it is not necessary forthe calculation of the distance from the source for the pitch to beconstant, but the periodic distribution of the imaging means, even ifthe period is not constant, simplifies the calculations making itpossible to calculate the distance. The linear array of micro-lenses isplaced in front of the linear array of detectors, parallel to it. Forexample, a CCD array 28 mm long comprising 2048 elementary detectors orpixels of 14 μm (standard format); it is then possible to choose to workwith a linear array of 100 micro-lenses of 287 μm diameter; each of thesub-pupils of the micro-lenses will then correspond to about 20elementary detectors on the detector; a focal length of 10 mm, forexample, will be chosen, which is relatively large with respect to thediameter of the micro-lens so that the profile of the light spotsremains within the diffraction limit; in the example described, thediffraction limit is about 50 μm, which corresponds to 3 or 4 elementarydetectors. FIG. 3 shows the profile of light spots obtainedexperimentally on the detector (in this particular case, a CCD array) .The micro-lenses may, for example, be spherical or cylindrical; in thiscase, the generatrices of the cylindrical lenses are substantiallyparallel and the axis of the linear array of detectors is perpendicularto that of the generatrices.

FIG. 2 describes a variant of the device comprising an optical system 10for shaping the beam. This makes it possible to define a nominalposition of the source to be measured (at the focal point of thissystem), to which corresponds a specific distribution of the lightspots, and its benefit in calculating the distance from the source willbe explained in what follows. This optical system may be a simple lensor a combination of lenses. It may moreover comprise a cylindrical lensmaking it possible to condense the light onto a linear array ofdetectors (since the latter is generally not very wide: a few microns toa few hundreds of microns); this configuration is particularlybeneficial in the case in which the system uses a linear array ofcylindrical lenses as an imaging means.

According to one variant, it is also possible to use a linear array ofcylindrical micro-lenses placed in front of a matrix of elementarydetectors. Each micro-lens then forms, from the light source, a lineparallel to the axis of the generatrices on the matrix of detectors. Thesignal can then be summed along the line to enhance the signal-to-noiseratio.

The imaging means may, according to another variant, be a matrix ofspherical or aspherical micro-lenses forming a set of light spots on amatrix of elementary detectors.

The device according to the invention further includes a calculatingcircuit making it possible, on the basis of the relative positions of atleast three light spots, to calculate at least one parametercharacteristic of the distance from the light source to the device.According to one variant, it further includes a circuit for spatialposition-fixing of the light sources on the detection means,determining, for each light spots, its position with respect to areference origin. FIGS. 4A, 4B and 4C make it possible to illustratepossible modes for calculating the distance from the source.

The position-fixing circuit makes it possible to determine the positionof the centre of each spot with respect to a reference origin. As far asthe calculation of the centre of each spot is concerned, varioustechniques exist: the spatial centre of gravity of the few elementarydetectors over which the spot is spread is calculated, for example, byassigning, to each elementary detector, a coefficient relating to itslevel of illumination; the centre of gravity then constitutes the centreof the spot and is determined to within a fraction of width of theelementary detector. It is also possible to interpolate the spot by itstheoretical shape or by a mathematical function approximating to it (acardinal sine (sin(x)/x), a Gaussian function (exp(−x²), . . . ), atechnique which is known and practised in the astronomy field. Theposition of the centre of each of the light spots can be measured byconsidering various origins: with respect to a fixed point of thedetector or with respect to the end or to the centre of each of thegeometric projections of the corresponding sub-pupils on the detector(in this case, the origin is said to be “floating”).

In order to explain possible modes of calculating the distance from thesource, three imaging means are considered, for example threemicro-lenses side-by-side extracted from the central part of a lineararray of micro-lenses and forming three sub-pupils of the device,referenced in FIG. 4A by SP₁, SP₂ and SP₃. The micro-lenses are assumedto be distributed with a substantially constant pitch. The light source4, assumed to be point-like or nearly point-like, sends rays into thedevice from successively any positions A and B (A and B lie in themeasurement field explained below). The rays coming from the lightsource pass through the plane 5 of the micro-lenses. Consequently, thebeam is broken down along the corresponding sub-pupils. The lensesre-image the source in the plane of the detector 7 for a nominalposition of the source (there is then, in this particular case,conjugation between the source and the detector) and in a plane in thevicinity of it for an other position (there is defocusing in the planeof the detector). In both cases, a set of light spots are formed on thedetector, which are distributed linearly in the example chosen, each ofthem corresponding to one sub-pupil. The defocusing is not troublesomesince, with the imaging means being open very little, they operate inthe diffraction regime and do not carry out imaging, that is to say thatthe dimensions of the light spot correspond to those of the diffractionspot of the imaging means.

These light spots are centred on the rays originating from the sourceand passing through the optical centre O_(n) of the micro-lenscorresponding to a sub-pupil SP_(n) as illustrated in FIG. 4A (the raysin solid line relate to position A and those of position B are in dottedline). In this Figure, the defocusing has been deliberately ignored (Aand B are re-imaged in the plane of the detector) the only consequenceof which is a more or less extensive spreading of the light spot (but itremains centred on the ray defined above). In the particular case setout in FIG. 4A, the position of the light spot corresponding to the subpupil SP₂ is unvarying, since the optical axis of the lens inscribedwithin SP₂ is coincident with the axis of movement of the light source(axis defined by (AB)). When using the device in practice, the centralsub-pupil is not necessarily well centred, but it is nevertheless closeto the centre and consequently the position of the light spot changesvery little.

FIG. 4A makes it possible to show one advantage of the optical shapingsystem 10 illustrated by FIG. 2. In fact it makes it possible, on theone hand, to be able to adapt the working distance easily (by changingthe optical system) and, on the other hand, to be able to have availablethe entire possible dynamic range of the system. In fact, FIG. 4A showsthat, with no shaping objective lens, the light spot will always beoff-centre and half of the sub-pupil remains inaccessible to it. Incontrast, FIG. 1 shows that, at the nominal distance, the light spot iscentred in the sub-pupil (focal point of the lens). When the opticalshaping system 10 is omitted, the matrix of lenses then alone anddirectly carries out both breaking-down of the beam into its sub-pupilsand focusing of it into light spots on the detectors.

One calculating mode consists in determining the average spatialseparation separating the successive light spots two-by-two. Thisapproach is possible since, in many particular instances of use, thelight spots are equidistant from one another (which can be likened to alinear distribution of the positions of the light spots). This is thecase when the imaging means are substantially identical and distributedwith a near-constant pitch. This separation is a parametercharacteristic of the distance from the source to the device. This isbecause, if the separations between two successive light spots arecalled dA and dB, for the positions A and B of the source respectively,then, according to the example of FIG. 4A:${d_{A} = {\frac{\lbrack{AD}\rbrack}{\left. {AD} \right\rbrack - \left\lbrack {DP} \right.}\left\lbrack {O_{1}O_{2}} \right\rbrack}},{{{and}\quad d_{B}} = {\frac{\lbrack{BD}\rbrack}{\left. {BD} \right\rbrack - \lbrack{DP}\rbrack}\left\lbrack {O_{1}O_{2}} \right\rbrack}}$

[O₁O₂]=p is the pitch of the sub-pupils and [DP]=f, the distanceseparating the plane of sub-pupils from the detector, is a geometricparameter of the system. Hence it is possible to extract the parameters[AD] and [BD) to be determined:$\lbrack{AD}\rbrack = {{\frac{f \times d_{A}}{d_{A} - p}\quad {{and}\quad\lbrack{BD}\rbrack}} = \frac{f \times d_{B}}{d_{B} - p}}$

This approach reveals the redundancy of the information supplied by thesystem (dA is measured (N−1) times and dB (N−1) times if N is the numberof spots).

One practical way of calculating this average separation consists indetermining the slope of the straight line representing the position ofthe centre of each spot as a function of the corresponding sub-pupil,indexed, for example, by a number (the sub-pupil 0 or 1 being chosenequally from among the set of sub-pupils). FIG. 4B illustrates thedistribution along a straight line of the points defined by the pairs ofpositions of each sub-pupil and positions of the centre of each of thelight spots. The dispersion about this straight line is related todefects in the system, especially to defects in manufacturing the matrixof micro-lenses. The calculation of the slope of the straight line canbe done, for example, by a conventional linear-regression method(least-squares method): the sum of the squares of the separationsbetween the measurements and the corresponding values of the estimatestraight line are minimised (the least-squares straight line is thusobtained).

FIG. 4C details the distribution of energy over the detector in thesub-pupils SP₁ and SP₂ of FIG. 4A. The distances separating the centresof the light spots formed (thick line for the source placed at A andfine line for the source placed at B) differ because of the longitudinaldisplacement of the source. This figure makes it possible to understandwhy the system can tolerate defocusing, even substantial: the spot willbe more spread out, but its centre will remain unchanged. Moreover, thelow numerical aperture of the imaging means and the diffraction regimelimit the significance of the defocusing.

The functions explained above show that the separation measured is aone-to-one function of the longitudinal position of the source. The sameis true for the slope. The definition of the longitudinal field ofmeasurement, or depth of field, depends on the geometrical parameterschosen (number of sub-pupils, diameter and focal length of the lenses).it is necessary for all the light spots formed on the detector to remainusable, that is to say for each of the light spots to retain asatisfactory profile (not too much aberration and no excessivedefocusing).

As far as the field of measurement is concerned, it is possible moregenerally to define a measurement volume. The light source should liewithin this volume in order to be able to be measured. The said volumeis defined as a function of the position and of the quality of the lightspots: along the X axis (see FIG. 4A), it is necessary for the lightspots to remain on the detector; along the Y axis, it is necessary forthe light spots to remain of good quality and to be in sufficientnumbers (they are imaged onto the adjacent sub-pupils, thereforeoff-axis); along the Z axis, it is a question of the depth of field,discussed above. Consequently, the measurement volume changes as afunction of the characteristics of the system.

The functions explained above show how, on the basis of knowledge of thegeometric parameters of the device, it is possible directly to determinethe distance from the light source to the device. In practice, thecalculating circuit may include a calibration table containing at leastone calibration parameter (for example average separation or slope ofthe straight line as they were discussed above), the values of which aredetermined for certain distances from the light source. These values canbe determined in advance in an experimental or theoretical manner. Itthen compares the parameter characteristic of the distance determined inthe course of the measurement with the values of the calibrationparameter. FIG. 5 describes the stages of a mode of operation of adevice according to the invention including a position-fixing circuit 51and a calculating circuit 52 including a calibration table 53, thecalculating circuit comparing the average separation or the slope whichare determined in the course of the measurement with the values of thecalibration parameters in order to extract therefrom the value of thedistance from the source.

The foregoing examples have dealt with light spots arranged linearly. Ina more general way, each imaging means being indexed, the distance fromthe light source can be determined on the basis of a parametercharacteristic of the curve representing the position function of eachlight spot as a function of the indices of the imaging means. Acalibration table containing calibration parameters determined forcertain distances from the source may here again be used by thecalculating circuit in order to extract the distance from the lightsource.

According to another method of operation of the device according to theinvention, it is not necessary, by means of the position-fixing circuit,to determine the position of the light spots. The calculating circuitmay carry out a spatial-frequency analysis of the distribution of thelight spots, the parameters characteristic of the distance from thelight source then being determined on the basis of the frequencyspectrum. For example, if the set of light spots are distributedsubstantially periodically, one of the parameters determined on thebasis of the frequency spectrum is then the inverse of the period. Thefrequency analysis can be carried out, for example, by means ofconventional fast-Fourier-transform algorithms.

FIGS. 6A and 6B represent a variant of the device described aboveapplied to the simultaneous measurement of the distances from severalpoint-like light sources. More particularly, what is involved is a setof light sources forming a line.

The light emitter 1 and its objective lens 2 are not necessarily alignedwith the optical axis 9 of the receiving system. The secondary lightsource is created by the transmitter of light projected along a lightline 4 and 4bis. FIG. 6A (top view) is a view in a plane perpendicularto the projected line 4; FIG. 6B (side view) represents the system in aplane containing the projected line 4bis. The detector used may be amatrix CCD camera 7.

The light coming back is picked up by an optical shaping system 10. Thebeam is then broken down (as in the case of the first device described)in the plane 5 of the sub-pupils along the sub-pupils 6, then focusedonto the detector 7. FIG. 6A reveals the similarity in measurementbetween a device measuring the distance from a point-like and nearlypoint-like light source and a device measuring the distance from thelight source consisting of a set of point-like or nearly point-likesources arranged into a line. In each plane perpendicular to the line,the same elements of the first device described are again encountered: alight source (intersection of the line and of the perpendicular plane inquestion), a linear array of imaging means, for example micro-lenses,the equivalent of a linear detector (the line 11 of the matrix detectorin the perpendicular plane in question): the distance separating thesaid light source from the device is a function of the separationmeasured between the centres of gravity of the corresponding light spotslying on the said linear detector (line 11 of the matrix detector 7). Asupplementary implementation constraint should nevertheless be takeninto account: the projected light line should be conjugated or close toconjugation with the detector in the plane orthogonal to the directionof the linear array of the imaging means. If this is not the case, thespatial resolution is degraded. The spatial resolution of such a device,that is to say the number of measurement points obtained on the profile,is a function of the number of elementary detectors in the directionparallel to the projected light line, as well as of its defocusing.

The device according to the invention is intended to be substituted foran entire range of sensors (in particular triangulation sensors) in theindustrial field for checking dimensions. Moreover, this device opens upnovel fields of application in areas such as industrial vision (fornon-destructive checking), biology (for example, following thetrajectory of a fluorescent object), robotics, etc.

What is claimed:
 1. Device for contactless measurement of distance froma light source including an optical detection means (7) formed byelementary detectors and a set of N imaging means (6), N≧3,characterized in that: the imaging means make it possible to image thelight source on a plane in the vicinity of that of the detection means,thus forming, on the detection means, a set of at least three lightspots, each spot being spread over at least two elementary detectors;the device further includes a calculating circuit making it possible, onthe basis of the relative positions of the at least three light spots,to calculate at least one parameter characteristic of the distance fromthe light source to the device with an accuracy depending on the numberN of light spots.
 2. Measuring device according to claim 1,characterised in that the calculating circuit includes a calibrationtable containing at least one calibration parameter the values of whichare determined for certain distances from the light source and comparesthe characteristic parameter with the values of the said calibrationparameter.
 3. Device according to claim 1, characterised in that itfurther includes a circuit for spatial position-fixing of the lightspots on the detection means, determining, for each light spot, itsposition (dA) with respect to a reference origin.
 4. Measuring deviceaccording to claim 3, characterised in that: each imaging means isindexed (SP1, SP2, etc.); the parameter characteristic of the distancecalculated by the calculating circuit is at least one of the parameterscharacteristic of the curve representing the position function of eachlight spot (dA) as a function of the indices of the imaging means(dA=f(SP1, SP2, . . . ).
 5. Measuring device according to claim 4,characterized in that the positions of the light spots determined by theposition-fixing circuit are substantially distributed along a line, thesaid curve is then substantially a straight line and the parametercalculated by the calculating circuit is the slope of the straight line.6. Measuring device according to claim 1, characterised in that thecalculating circuit carries out a spatial frequency analysis of thedistribution of the light spots, the parameters characteristic of thedistance from the light source then being determined on the basis of thefrequency spectrum.
 7. Measuring device according to claim 6,characterised in that the set of light spots are distributedsubstantially periodically, one of the parameters determined on thebasis of the frequency spectrum then being the inverse of the period. 8.Measuring device according to claim 1, characterised in that the imagingmeans consist of a linear array of cylindrical micro-lenses,substantially identical, arranged side-by-side with a substantiallyconstant pitch, the generatrices of the cylinders being substantiallyparallel, and in that the detection means are a matrix of elementarydetectors or a linear array of elementary detectors the axis of which isperpendicular to the generatrices of the micro-lenses.
 9. Measuringdevice according to claim 1, characterised in that the said light sourceconsists of an illuminated elementary surface of an object, thus forminga secondary light source, and in that it includes an illumination systemmaking it possible to create the said secondary light source. 10.Measuring device according to claim 9, characterised in that it includesa beam splitter placed between the secondary light source and theimaging means, the splitter directing the light originating from theillumination system onto the said elementary surface and transmitting,to the imaging means, the light originating from the secondary lightsource, making it possible to make the axis of the illumination systemand the optical axis of the device collinear.
 11. Measuring deviceaccording to claim 1, further comprising a shaping optical system placedbetween the light source and the imaging means for adapting the fieldfor measuring distance from the light source.
 12. Device for contactlessmeasurement of distance from a light source comprising: an opticaldetector comprising a plurality of elementary detectors; N imagingelements, where N≧3, the imaging elements being arranged with respect tothe optical detector so that light from the light source can be focusedon a plane in a vicinity of the optical detector to form at least threelight spots, each of the spots being spread over at least two of theelementary detectors; and a calculating circuit receiving as an inputinformation related to relative positions of the at least three lightspots, the calculating circuit producing as an output datarepresentative of at least one parameter characteristic of a distancefrom the light source to the device.
 13. The device of claim 12, whereinthe calculating circuit is constructed to perform spatial frequencyanalysis of distribution of the light spots.